A market has developed for inkjet printers that can dock directly with digital cameras. These printers are expected to download and print the photographs quickly and with a minimum of user input. A significant part of the delay between docking the camera and printing the photographs, is the processing time needed to prepare the image data for the printhead.
The major processing tasks are JPEG decompression, colour space conversion, image rotation and halftoning to convert the colour values into dots. These tasks are briefly explained below.
Colour can be specified using three independent variables. The variables are essentially coordinates in a colour space. The same colour can be specified in different colour spaces using different variables. Each colour space has a particular use or application. RGB (red, green, blue) is the natural colour space for devices that emit light such as television screens or computer monitors. CMY (cyan, magenta, yellow) is the natural colour space for devices that display images with reflected light such as printed images from a printer. YCRCB (luminance, chrominance red, chrominance blue) separates the luminance from the chrominance (usually abbreviated to ‘chroma’) channels for more convenient data compression. Humans are more sensitive to luminance than chroma so any change in chrominance resulting from compression and subsequent decompression will be less noticeable than an equivalent change in luminance. This means both chroma channels can be heavily compressed as long as the luminance is lightly compressed. With two of the three channels heavily compressed, YCC image data is more efficiently handled by the processor.
Digital cameras capture images natively in RGB. For efficient storage, the images are converted to YCC and compressed. Image data downloaded from a camera is typically in sYCC which is a widely recognized standard form of YCC. This must then be colour space converted when it is output to a screen or printer.
If the image is downloaded to a printer, the data is converted to the printer's colour space and the separate colour channels are halftoned with a dither matrix. Halftoning exploits the eye's perception of a spatial average of printed dots to reproduce contone (continuous tone) images. Inkjet printers can either print a dot at any one of the addressable locations on the media, or not. However, dots dispersed over area of white (say) paper, will appear to eye as a contone shade somewhere between white and the dot colour, depending on the number of dots.
The dither matrix covers a small area of the image at a time. The matrix has a range of threshold values dispersed throughout its sites. The contone colour levels for each pixel are compared to the spatially corresponding threshold values within the matrix. If the contone level exceeds the threshold value, a dot of that colour is printed (or equivalently, a dot is printed if the contone level is greater than or equal to the threshold, or less than, or less than or equal to the threshold value). This will produce many micro-differences between the contone and halftone image, but the eye is largely insensitive to these high frequency differences.
To produce a colour image, the separate halftoned images for each of the three colour channels are superimposed by the printer on the media. Printers typically have cyan, magenta, yellow and sometimes black (to conserve the other inks and provide a ‘truer’ black). This is abbreviated to CMYK (Cyan, Magenta, Yellow and blacK). If the print resolution, or dots per inch (dpi) is high enough, halftoning can reproduce any colour in the printer's gamut (palette of printable colours). Accordingly, the individual dots in CMY(K) space are colour averaged by the eye to reproduce the colours of the original image.
Downloaded images can be manipulated and enhanced on a computer prior to printing. However, if downloaded directly from a camera to a photo printer, the user does not have an opportunity to manually enhance and view the images prior to printing. Even so, it is possible to incorporate some relatively basic image enhancement into the printer that can be automatically and uniformly applied to the images prior to printing.
One common and relatively basic image enhancement technique is histogram expansion. It improves the colour contrast by expanding the range of colours present in the raw image data so that it is more evenly spread across the entire range of available colours. To do this, it is necessary to collect image statistics and build histograms for each colour channel. This involves the collection of the three colour levels for each pixel and recording the number of pixels that fall into a range of discrete colour level intervals to build the histograms. Usually the original image will have histograms with at least one sparsely populated region. By re-assigning all the pixels in the sparsely populated region into one of the colour levels, the rest of the histogram can expand into the vacated region. Spreading the pixels more evenly across the histogram improves the colour contrast. As the number of pixels in the sparsely populated regions is insignificant, reassigning them to a single colour level has little, if any, detrimental effect on the image. So in the vast majority of cases, the net effect of histogram expansion is an enhancement of the image.
Unfortunately, the processing tasks associated with histogram expansion can delay printing. In some printing applications, there is an expectation that the printer will start printing the downloaded images almost instantly. Photograph printers that dock directly with the camera are one such example. The captured images will typically download from the camera upon docking and automatically print the images to 6 inch by 4 inch photo grade paper. Users prefer, if not expect, to see their photos being printed within a few seconds. More importantly, users expect good quality prints, but, as discussed above, computationally intensive image enhancement is counter to quickly initiating the printing of downloaded photos.